Fix webpacker upgrade issues: jets build fails for new project

[undefinedvalue]

Checklist

• [x] Upgrade Jets: Are you using the latest version of Jets? This allows Jets to fix issues fast. There's a jets upgrade command that makes this a simple task. There's also an Upgrading Guide: http://rubyonjets.com/docs/upgrading/
• [x] Reproducibility: Are you reporting a bug others will be able to reproduce and not asking a question. If you're unsure or want to ask a question, do so on https://community.rubyonjets.com
• [x] Code sample: Have you put together a code sample to reproduce the issue and make it available? Code samples help speed up fixes dramatically. If it's an easily reproducible issue, then code samples are not needed. If you're unsure, please include a code sample.

My Environment

Software	Version
Operating System	OSX 10.13.4
Jets	1.8.8
Ruby	2.5.3

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Expected Behaviour

A newly created Jets project should build successfully.

Current Behavior

A newly created Jets project fails to build due to webpack errors.

Here is the output of jets build err.txt

Step-by-step reproduction instructions

gem install jets
nvm install 8.10.0
nvm alias default node
npm install -g yarn
jets new --no-database demo
cd demo
jets build

[tongueroo]

@undefinedvalue Thanks for the report. This was an important one. Release fix in v1.8.9

Had to dig into this one a bit. Some notes for posterity:

• Webpacker upgraded to a newer version of babel.
• Looks like babel renamed a few of its libraries to be under the @babel namespace. IE: syntax-dynamic-import -> @babel/plugin-syntax-dynamic-import. So needed to update the generated .babelrc file to the new libraries.
• The postcss-cssnext plugin was also moved to a separate library. So that needed to be added as part of the jets new process.
• Webpacker also change the default behavior so that css files do not get compiled. You must now turn it on with extract_css: true in your webpacker.yml

Big thanks to: https://github.com/babel/babel-upgrade for helping with the fix.

Pushed out a release to fix this quickly. Would like to revisit the tongueroo/webpacker fork for Jets and release it as a separate gem when got time. Unsure when. Will consider PRs.